Thanks dwig. It seems to me to be mostly like Mardown but with customisations for certain things like not using double * or _ for emphasis and using # for bold. It doesn't recognise ### Header 3 for a third level header.

AZW is Amazon's name for their MOBI format. Check the MOBI documentation.

Dale

The OP is inquiring about documentation for the input format (TXT with markup) and not the output format (AZW, MOBI, ...)

Amazon has a daemon which converts emailed TXT files to AZW for delivery to your Kindle. This daemon appears to respect some markup in the source TXT but there doesn't seem to be documentation, or at least easily discovered documentation, for the markup supported by the daemon.